#1. Edit the redirect-register page to look like a payment/register page. Ofcourse you need to know extra coding that involves payment stuff, like a shopping cart script, although your members aren't really buying anything.
#2. If all existing registered members are willing to pay the fee, I don't see any reason why you would have to reset them. You just have to add them to this "list" of members who will be fee'd yearly. Maybe you could create an extra column in the user table, and call it "paid membership", and have members with a paid membership will have this field set to true. Then again every member on your forum will be a payed member, so I guess this isn't needed.
Once again you need extra coding knowledge on ebusiness type stuff which isn't really given at all on VB.org, probably a site like Sitepoint.com you could learn some of it.
That's my input, I maybe wrong or right in someways, but there you have it